OBJECTIVE: To establish a guideline to predict viability and functional recovery of hypoperfused myocardium. METHOD: An excised rabbit heart will be perfused with oxygenated blood via a retrograde aortic cannula. The net developed tension (T Nmax) of the papillary muscle will be measured after the papillary muscle is detached from the mitral annulus. During and after hypoperfusion of varying degrees and duration, T Nmax will be monitored. At the same time subendocardial pH, epicardial ECG, serum levels of pyruvate, lactate, myocardial specific isoenzymes, and electrolyte concentrations of the coronary sinus blood will be determined before, during, and after hypoperfusion. Myocardial tissue enzymes will be determined subsequently. Histochemical, light and electron microscope studies will be performed on the hypoperfused myocardium and will be correlated with the above parameters. The functional recovery of the hypoperfused myocardium will be correlated with the duration of hypoperfusion, T Nmax, subendocardial pH, and ECG during hypoperfusion and serum enzyme levels after reperfusion at normal pressures.